1. Field of the Invention
The present invention relates to an acceleration sensor and a method of manufacturing the same.
2. Description of Related Art
A sensor to which an MEMS (Micro Electro Mechanical Systems) technique is applied has been recently loaded on a portable telephone, and hence an MEMS sensor is increasingly watched with interest. For example, an acceleration sensor for detecting the acceleration of a substance is known as a typical MEMS sensor.
FIG. 13 is a schematic perspective view of a conventional acceleration sensor.
An acceleration sensor 101 is a piezoresistive sensor detecting acceleration with piezoresistive elements, and includes a sensor chip 102 and glass chips 103 and 109 for sealing (device-sealing) the sensor chip 102.
The sensor chip 102 includes a frame 104, a weight 105 and four beams 106.
The frame 104 is in the form of a quadrangular ring (a frame) in plan view.
The weight 105 is arranged on a region surrounded by the frame 104 at an interval from the frame 104. The weight 105 is formed by a central columnar portion 107 in the form of a quadrangular column and four peripheral columnar portions 108 in the form of quadrangular columns provided on the periphery of the central columnar portion 107. Each of the central columnar portion 107 and the peripheral columnar portions 108 has a thickness (height) identical to that of the frame 104. The central columnar portion 107 is arranged on a central portion of the region surrounded by the frame 104, so that the outer peripheral edges thereof are parallel to the inner peripheral edges (the inner surfaces) of the frame 104 in plan view. The peripheral columnar portions 108 are arranged one by one on extensions of diagonal lines on the upper surface of the central columnar portion 107. Single corners of the side surfaces of the peripheral columnar portions 108 are connected to the corners of the side surfaces of the central columnar portion 107 respectively. Thus, the central columnar portion 107 and the four peripheral columnar portions 108 integrally constitute the weight 105 having the same thickness as the frame 104.
Each beam 106 extends between each pair of peripheral columnar portions 108 adjacent to each other, parallelly to the side surfaces of the peripheral columnar portions 108 at intervals. An end of the beam 106 is connected to the frame 104, while another end thereof is connected to the central columnar portion 107. The beam 106 has a thickness of about 7 μm, for example, to be deformable and deflectable due to the thickness. Thus, the four beams 106 support the weight 105 to be vibratile on the frame 104.
The glass chip 103 is in the form of a quadrangular plate having outer peripheral edges generally identical in shape to the outer peripheral edges (the outer surfaces) of the frame 104 in plan view. The glass chip 103 is made of heat-resistant glass such as Pyrex (registered trademark), for example, and anodically bonded to the lower surface of the sensor chip 102.
The glass chip 109 is also in the form of a quadrangular plate having outer peripheral edges generally identical in shape to the outer peripheral edges (the outer surfaces) of the frame 104 in plan view. The glass chip 109 is made of heat-resistant glass such as Pyrex (registered trademark), for example, and bonded to the upper surface of the sensor chip 102.
A plurality of piezoresistive elements (not shown) are arranged on the four beams 106.
When acceleration acts on the acceleration sensor 101 and the weight 105 vibrates, the beams 106 are distorted. Due to the distortion of the beams 106, stress acts on the piezoresistive elements provided on the beams 106, to change the resistivity of the piezoresistive elements. When the change of the resistivity of each piezoresistive element is extracted as a signal, therefore, the physical quantity (acceleration) acting on the acceleration sensor 101 (the weight 105) can be detected on the basis of the signal.
In the acceleration sensor 101, the glass chips 103 and 109 are bonded to the sensor chip 102, thereby sealing (device-sealing) the sensor chip 102. A space for holding the weight 105 in a vibratile manner is formed due to the device sealing.
In order to bond the sensor chip 102 and the glass chips 103 and 109 to one another, the sensor chip 102 and the glass chip 103 are first anodically bonded to each other.
In order to anodically bond the sensor chip 102 and the glass chip 103 to each other, the sensor chip 102 is aligned with each of glass chips 103 arrayed on a wafer-type glass substrate. In this case, the outer peripheral edges of the sensor chip 102 are aligned with alignment marks formed on the glass chip 103 while the position of the sensor chip 102 with respect to the alignment marks is confirmed (visually recognized) through the glass chip 103. Then, the lower surface 104 of the frame 104 of the sensor chip 102 and one surface of the glass chip 103 are approximated to each other, and a high voltage is applied therebetween. Charge is formed in the vicinity of the lower surface of the frame 104 and the surface of the glass chip 103 due to the application of the high voltage, and an electric double layer is formed through the interface therebetween. Thus, the sensor chip 102 and the glass chip 103 are anodically bonded to each other.
After the sensor chip 102 and the glass chip 103 are bonded to each other, the glass chip 109 is aligned with the glass chip 103 while the alignment marks are confirmed (visually recognized) from the side of the other surface of the glass chip 103 through the glass chip 103, and bonded to the sensor chip 102.
On the other hand, there is a demand for substituting low-priced silicon chips for the glass chips 103 and 109 as the chips for device-sealing the sensor chip 102. However, the silicon chips are not excellent in transparency, dissimilarly to the glass chips. When the lower silicon chip is bonded to the lower surface of the sensor chip 102 by aligning the outer peripheral edges of the sensor chip 102 with alignment marks formed on the silicon chip, therefore, the alignment marks cannot be visually recognized from above and from under the silicon chip. Therefore, it is difficult to align the upper silicon chip with the lower silicon chip, and the positioning accuracy of the upper silicon chip with respect to the lower silicon chip is disadvantageously reduced.